WASHINGTON – In the strongest sign of progress on federal gun control since the December school shootings in Newtown, Conn., a bipartisan group of senators, including Kirsten E. Gillibrand of New York, on Monday announced an agreement that would, for the first time, make gun trafficking a federal crime.

The proposal, which the Senate Judiciary Committee will consider later this week, will include criminal sanctions against straw purchasers, who buy weapons for people who are barred by law from doing so. Straw purchasing is a long-standing problem that has led to guns reaching states with tough gun-control laws such as New York.

“In New York, nine out of 10 guns used in crimes are criminally trafficked, illegal weapons,” said Gillibrand, a Democrat who has been proposing federal gun-trafficking legislation since 2009, her first year in the Senate. “This bill is the best tool to begin to crack down on that.”

While bipartisan legislation often hits fatal snags in the Senate, where 60 votes are required for most important measures to pass, Gillibrand said she feels good about the gun-trafficking bill’s prospects.

In another sign that the bipartisan gun-trafficking proposal may move forward, the top Republican on the Judiciary Committee, Sen. Charles Grassley of Iowa, indicated last week that he had been working with Gillibrand on the then-evolving deal.

“She and I have had good conversations about modifying her bill to respond to my concerns, and I’ve had discussions with her as we’ve been voting on the Senate floor on other legislation, and I find that she’s very satisfied with suggestions that we’ve made,” Grassley said last week on the Senate floor.

The bill prohibits gun trafficking by making it a federal crime to ship or otherwise transfer two or more guns to anyone known to be barred by law from owning them, such as convicted felons and the criminally insane. Violators of the law would be subject to prison sentences of up to 15 years – or 25 years for the leader of any gun-trafficking ring.

Law enforcement agencies have long sought such legislation, saying current law – which targets only people making false statements to federally registered gun dealers – is too narrow and penalties are too paltry to deter gun trafficking and straw purchases.

“Every year, we see hundreds of thousands of guns diverted from legitimate commerce to the criminal marketplace, all because our weak federal laws treat gun trafficking as little more than a paperwork violation,” said Mark Glaze, director of Mayors Against Illegal Guns.

Gillibrand said she expected support for the bill to swell this week, and Leahy agreed.

“I hope that as other senators on both sides of the aisle become more familiar with our bipartisan proposal, they will understand how it provides law enforcement with the tools they need to go after those who engage in the straw purchasing and illegal trafficking of firearms,” Leahy said.

In addition to Kirk, Sen. Susan Collins, R-Maine, signed on as a co-sponsor of the gun-trafficking legislation.

Similar legislation has been introduced in the House, and Gillibrand spoke with House Majority Leader Eric Cantor, R-Va., about the legislation over the weekend, when they both attended the anniversary of the historic 1965 civil rights march in Selma, Ala.

While the House Judiciary Committee has not yet taken up the measure, the House bill, like its Senate counterpart, has bipartisan support.

The gun-trafficking bill is one of several that senators from both parties have been privately discussing in recent weeks.

Of those other efforts, the most far-ranging would be an expansion of background checks to all gun purchases, including those made at gun shows where, in many states, people can buy and sell weapons without any government review.

Sen. Charles E. Schumer, D-N.Y., is negotiating the details of that bill with Sen. Joe Manchin, D-W.Va.; Sen. Tom Coburn, R-Okla.; and Kirk, but no agreement has been announced.